made some changes to Course Instructor model and solve some migration issues

[hn2006]

Issue that this pull request solves

Closes: # (issue number)

Proposed changes

change in Course Instructor model

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (please describe):

Checklist

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• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have created new branch for this pull request
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• My changes does not break the current system and it passes all the current test cases.

Screenshots

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Other information

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